1. Technical Field
This invention relates to digital communications systems and, more particular, to a method and apparatus for detecting and matching a data transmission rate between devices within such systems.
2. Description of the Prior Art
With the advent of large databases allowing information retrieval services, there has been a proliferation of various terminals, and computer configurable as terminals, in both businesses and homes for use with these databases.
Communication with a database is usually obtained through use of a data module which connects a terminal to the database through the telephone switched network. Data communications between the data module and the terminal may be in either of the well known bit synchronous or character asynchronous formats.
When a terminal operating in the asynchronous format is connected to a data module, seldom is the data going between the device initially at the same speed. Rather, switches must often be manipulated on both devices in some systems to achieve the proper data speed match.
One approach in some systems has been to provide speed matching in the data module wherein the data module adapts to the speed of the connected terminal. Generally in this arrangement, the speed matching is accomplished by having a processor in the data module scan the received data lead very rapidly. This allows the processor to determine the speed of the character being transmitted by the terminal and match the speed of the data module thereto. Unfortunately, many other activities must be performed by the processor during the speed matching period of a data telephone employed in an Integrated Service Digital Network (ISDN) or a Digital Communications Protocol (DCP). In systems employing these protocols, the processor is therefore not normally available to do this sampling of data in order to determine the data transmission rate.
Another approach known in the art is to use the synchronous mode of a universal synchronous asynchronous receiver transmitter (USART) to match the speed of data from the terminal. The terminal receiver is placed in a bit synchronous receive mode by an associated processor using a specific predetermined control character. After determining the speed, the USART is placed back into the asynchronous mode to recover further characters. While the speed of transmission can be detected, this approach cannot reliably receive the first character, or may have problems receiving a second character if it arrives at a high speed with no additional delay. This approach also requires the use of a USART which is costly and therefore less acceptable where low cost in a system is desirable.